1. Field of the Invention
The present invention relates in general to radio-over-fiber (RoF) systems, and in particular relates to transponders used in RoF systems.
2. Technical Background
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, so-called “wireless fidelity” or “WiFi” systems and wireless local area networks (WLANs) are being deployed in many different types of areas (coffee shops, airports, hospitals, libraries, etc.). The typical wireless communication system has a head-end station connected to an access point device via a wire cable. The access point device includes digital information processing electronics and a RF transmitter/receiver operably connected to an antenna. The access point device communicates with wireless devices called “clients,” which must reside within the wireless range or a “cell coverage area” in order to communicate with the access point device.
The size of a given cell is determined by the amount of RF power transmitted by the access point device, the receiver sensitivity, antenna gain, and the RF environment, as well as by the RF transmitter/receiver sensitivity of the wireless client device. Client devices usually have a fixed RF receive sensitivity, so that the above-mentioned properties of the access point device largely determine the cell size. Connecting a number of access point devices to the head-end controller creates an array of cells that provide cellular coverage over an extended region.
One approach to deploying a wireless communication system involves creating “picocells,” which are wireless cells having a radius in the range from about a few meters up to about 20 meters. Because a picocell covers a small area, there are typically only a few users (clients) per picocell. A closely packed picocellular array provides high per-user data-throughput over the picocellular coverage area. Picocells also allow for selective wireless coverage of small regions that otherwise would have poor signal strength when covered by larger cells created by conventional base stations.
One type of wireless system for creating picocells utilizes radio-frequency (RF) signals sent over optical fibers—called “radio over fiber” or “RoF” for short. Such systems include a head-end station optically coupled to a transponder via an optical fiber link. Unlike a conventional access point device, the transponder has no digital information processing capability. Rather, the digital processing capability resides in the head-end station. The transponder is transparent to the RF signals and simply converts incoming optical signals from the optical fiber link to electrical signals, which are then converted to radiated electromagnetic signals via an antenna. The antenna also receives radiated electromagnetic signals from one or more client devices and converts the radiated electromagnetic signals to electrical signals. The transponder then converts the electrical signals to optical signals, which are then sent to the head-end station via the optical fiber link.
Multiple transponders are typically distributed throughout an optical fiber cable as a “transponder array,” wherein the optical fiber cable carries optical fiber links optically coupled to the transponders. The picocells associated with the transponder array form a picocell coverage area. To reduce picocell cross-talk, high-directivity transponder antennas can be used.
One application of picocellular wireless systems involves providing a number of different services (e.g., wireless local area network (WLAN), voice, RFID tracking, temperature and/or light control) within a building, usually by deploying one or more optical fiber cables close to the ceiling. This requires the picocells to be directed downward from the optical fiber cable, which requires an efficient and compact directional transponder that operates at multiple frequencies.